Cavity wall insulation and outside rendering

I had a company round this week to do an energy survey on my house and see if I qualify for a grant to get cheaper cavity wall insulation…which I apparently I do! My house is a 1930’s end of terrace, which is new enough to have cavities but old enough not to have any insulation…

The guy doing the survey has told me that if they are to install the insulation then I will have to knock back 75mm of the outside render, above the damp proof course so I don’t get ‘cold spots’…is what he is telling me correct? His advice seems a bit arbitrary on the basis that all the houses around me have the outside render down tot he ground without such a gap….

Oh hive mind, give me the truth of the matter….

Pile of s h I t e

Read up on old houses, damp and breathing of brickwork , then don’t bother with the cavity stuff. Fundamentally, I’m of the principle that walls need to breathe and that if you block one sid of a wall, then it has to breathe through the other side. So if you fill the cavity the. The wall would have to breathe into the room instead … Making the inside wall more prone to moisture on it. If you leave the cavity then The breathing can be done into the cavity rather than the room.

That being said, I do understand the thought process behind cutting back render away from the ground. It’s been done to our farmhouse and I reckon it probably does help.

Cost me about four grand to have the cavity wall insulation removed from my 1950s house. Cured the damp though.

🙄 Just make sure you get a proper guarantee: either insurance-backed or from someone large and reputable like Rockwool. Blown mineral fibre should be fully breathable, so won’t increase the risk of condensation on the inside. I had it done to my house around 20 years ago and no issues since, just a warmer house 😀

Considering it for our house, but on’t most installers now use polystyrene beads rather than blown rockwool, which solves the issue of damp because the polystyrene won’t wick water like rockwool will?

This is an interesting thread! I’ve got a bit of a damp thing going on in a 1920s house built with engineering brick, with the polystyrene balls cavity insulation.

Is this actually a thing that’s been mis-sold to the previous owner? It’s easy enough for me to remove it whilst the ground floor is back to brick.

(Apologies for slight hijack)

Wow, I’m glad I asked….

Have found this website….whilst it sounds a bit like ‘scaremongering’, the comments at the end are very telling…

Cavity Wall Insulation Problems

Depends on a great many things. Hundreds of thousands of houses across the UK houses have had retrofit cavity insulation and are fine. Thousands aren’t. There’s a healthy industry for rectifying faults.

There problem is, IME, the installers. Any fool can buy the kit and slam insulation into people’s houses, but they should be addressing the properties carefully and giving the correct advice rather than chasing the cash.

I can’t think for one second that the cavity in a 30s property is anywhere near large enough to put insulation in without getting problems.

I think location is another important factor. I’m on the west coast of the Highlands, a couple of hundred years from the sea. My house gets a lot of rain. CWI was probably not appropriate for this house in this location.

I’ve had two houses with cavity wall insulation, polystyrene ball type and both houses have had (relatively minor) damp issues as a result.

I live in Brighton about 0.5 mile from the seafront, although I have buildings between my house and the sea, we still experience driving rain…it seems like this is one of the key risk factors with respect to ongoing issues with damp…

Thank you, oh hive mind 🙂

Perhaps we live in a similar property, I live in a 30’s place by hove lagoon. Can’t help with insultion but I did find the wall ties on the prevailing side of the house were rusted to shit